Magnesium alloys have the lowest specific gravity among practical metal materials, and therefore in recent years, they have increasingly been used for casings of portable equipment and raw materials for automobiles requiring more lightweight. As a current practical manufacturing method of the products, casting by injection molding of a magnesium alloy, such as die casting or thixotropic molding, has predominantly been used.
When products are formed from a magnesium alloy by casting such as die casting or thixotropic molding, casting defects, such as wrinkled surfaces and shrinkage cavities, tend to occur because the latent heat of magnesium per unit volume is low. To repair these defects, putty finishing or grinding, for example, may be required, which considerably decreases productivity and results in higher cost and higher price. In addition, since wrinkled surfaces, shrinkage cavities, or the like are liable to occur, thinning of the product is difficult to achieve. Furthermore, since products are manufactured without plastic working from materials produced by casting, there has been a problem in that it is difficult to improve the strength thereof.
Among the proposed methods, there have been methods in which a cast material obtained by semi-continuous casting such as direct-chill casting (hereinafter referred to as DC casting) is hot-extruded into a predetermined shape, and the extruded material is subjected to rolling process or the like to form a thinner sheet metal, from which shaped products are produced by presswork or the like, or the extruded material is directly formed into shaped products by forging or other method. However, in the case where a sheet for presswork or a material for forging is manufactured by semi-continuous casting such as DC casting, the grain size of a material produced by such casting method is large, and hence it is difficult to directly carry out its presswork or forging as it is. Accordingly, the grain size must be fined by reheating and hot extrusion of the material obtained by the semi-continuous casting. Since the above-described working process of hot extrusion of a cast material must be performed, the number of working processes is increased, which results in decreased productivity and high cost. In addition, since a magnesium alloy is an active metal, the extrusion must be performed at an extrusion rate at which sufficient cooling can be attained so that blackening of a surface thereof or burning may not be caused by heat generated due to processing.
Accordingly, there have been problems of an appreciable decrease in productivity, resulting in high cost, and high price. Additionally, a drawback of a hot extruded material is that it is difficult to process into a complicated shape since the grain size thereof is not sufficiently fine to form a complicated shape.